Identify variables and constants in each of the following expressions:

i)4πr^2, the area of the surface of a sphere with radius r.

ii)S(1 + PT/100), the amount of money obtained if rupees S is invested at simple interest P per cent per year for T years.

iii)1/2gt^2, the distance in centimetres that a freely falling object covers in t seconds, g being equal to 980 cm/sec^2.

iv)mMG/d^2, the force of attraction between two objects of mass m and M situated at a distance d apart, G being the gravitational constant.